The present invention relates generally to systems for measuring optical distortion in transparencies, and more particularly to a system for measuring angular deviation in transparencies such as aircraft and automobile windscreens, visors and optical lenses.
Existing systems for measuring optical angular deviation in transparencies include those described in or referenced by U.S. Pat. No. 4,249,823, U.S. Pat. No. 4,377,341 and U.S. Pat. No. 4,398,822. Teachings of these patents and background material referenced therein are incorporated herein by reference without reiteration. A method finding substantial use and upon which an ASTM (American Society of Testing and Materials) standard test method is based utilizes the teachings of the '341 patent. In the '341 patent an incoherent light source illuminates a target in the form of an opaque slide with a transparent L-shaped pattern therein. Images of the legs of the L-shaped pattern are projected through a transparency under test to measure horizontal and vertical components of angular deviation characterizing the transparency. The method based on the '341 patent depends on geometric separation of the vertical and horizontal angular deviation components and results in horizontal and vertical angular deviation measurements at different locations on the transparency due to off-set effects of using an L-shaped pattern. The precise area of the transparency under test is therefore no easily determinable and resulting vignetting effects at the receiver lens of the system can lead to errors sufficiently significant to produce unacceptable results.
The invention described herein substantially solves or reduces in critical importance problems with existing systems for measuring angular deviation in transparencies by providing a system in which the vertical and horizontal components of angular deviation may be separated for independent simultaneous measurement at a specific locus of the transparency. Two fundamental nonlimiting approaches to separately optically encoding information on the orthogonal components are described. Certain embodiments include a dichroic beam splitter or set of color filters to project wavelength separated (e.g. red and green) crossed line images of a test beam through a transparency under test in order to examine separately the orthogonal components of angular deviation characterizing a specific area of the transparency. In another embodiment horizontal and vertical components of the test beam are encoded with different polarization vectors.
It is therefore a principal object of the invention to provide a system for measuring angular deviation in a transparency.
It is a further object of the invention to provide an angular deviation measurement system wherein both the horizontal and vertical angular deviation components are measured simultaneously for a specific portion of a transparency.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.